As the use of computers and especially handheld or mobile electronic devices continues to increase at a rapid rate, the demand for peripherals and systems connected via wireless connections continues to increase. The number of wireless applications is currently increasing at a very high rate in areas such as security alarms, networking, personal computing, data communications, telephony and computer security.
Wireless communications currently may take many forms such as ultrasonic, IR and RF. In the case of RF communications, wireless transmitters, receivers and transceivers use one or more antenna elements to convert an electrical RF signal to and from an electromagnetic wave. During transmission, the antenna serves as a radiator, generating the electromagnetic wave. During reception, the antenna serves as an absorber, receiving the electromagnetic wave.
An antenna is a transducer designed to transmit and/or receive radio waves which are a class of electromagnetic waves. Antennas function to convert RF electrical currents into electromagnetic waves and to convert electromagnetic waves into RF currents. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, Wireless Local Area Network (WLAN), Broadband Wireless Access (BWA), radar, and space exploration.
An antenna typically comprises an arrangement of electrical conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current. When placed in an electromagnetic field, the field induces an alternating current in the antenna and a voltage is generated between its terminals.
An antenna is an electrical element having defined resonance frequencies and bandwidth. The resonant frequency of an antenna is related to the electrical length of the antenna (i.e. the physical length of the wire divided by its velocity factor). Typically, an antenna is tuned for a specific frequency and is effective for a range of frequencies usually centered around the resonant frequency. Other properties of the antenna (especially radiation pattern and impedance), however, change with frequency.
Often times, however, the natural operating frequency band of the antenna is either (1) not sufficiently wide enough to cover the needs of the wireless system or (2) is not in the proper spectrum location where the antenna is required to function. In such cases, a tuning circuit is added to the antenna which allows it to operate over a wide range of frequencies.
As an electromagnetic wave travels through the different parts of the antenna system (e.g., radio, feed line, antenna, free space) it encounters differences in impedance. At each interface, depending on the impedance mismatch, some fraction of the energy of the wave reflects back to the source forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave is called the standing wave ratio (SWR). Minimizing impedance differences at each interface via impedance matching minimizes the SWR and maximizes power transfer through the antenna system.
As with the resonant frequency, the complex impedance of an antenna is also related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, however, the impedance is adjusted at the load using an antenna tuning circuit, balun, matching transformer or impedance matching networks composed of inductors and capacitors.
Currently, two types of antenna tuning circuits are in widespread use today. One is varactor based tuning circuits and the second is PIN diode tuning circuits. Note that in some cases, the two circuit types can be combined into a single hybrid tuning circuit.